1. Field of the Invention
The present invention generally relates to ultraviolet sensors and, more particularly, to a sensor design which virtually eliminates solarization effects on optical components and photodetector.
2. Background Description
Ultraviolet (UV) radiation has been long known to produce solarization in materials. Solarization presents a major problem in UV measuring instruments because it changes the transmission characteristics of the measurement light path and hence, changes the reading produced by the instrument.
The degree of solarization is a function of, among other factors, how many UV photons strike the material. Since there is a finite probability that a single photon striking a piece of material will cause a physical change in the material, one key to reducing solarization is to reduce the number of photons striking the material in question. However, UV measurements require a finite number of photons striking a unit area to make a measurement.
The problem is a long standing problem. One attempt to solve the problem is disclosed in U.S. Pat. No. 5,514,871 to Hayes et al. The Hayes et al. UV sensor comprises an attenuating aperture, filter and photodetector serially disposed in the path of the UV radiation. If the light source covers a large viewing angle, the light source will be attenuated by the ratio of the size of the aperture divided by the height of the light source. However, if the light rays are collimated (parallel to each other), the aperture will pass the rays with unattenuated intensity. While the unattenuated rays would form an image only the size of the aperture, they would be unattenuated and the associated intensity would be sufficient to solarize the filters and detector in the area where the spot fell. Thus, the Hayes et al. approach works only for large area sources.